1. Field of the Invention
This invention concerns a molding die for use in casting which is utilized for casting processes such as die casting, pressure casting and vacuum casting and, more particularly, it relates to a molding die for casting metal articles by using high temperature molten metal at a temperature about from 600 to 1650.degree. C. and solidifying them under pressure.
2. Description of the Prior Art
In the casting process of this type, metal articles are cast by charging molten metal to the inside of a molding die comprising paired male and female dies, and cooling the molten metal under pressure to a temperature in a solidification region and solidification range where the solid phase and liquid phase are present together while applying a pressure to the molten metal. Since the control for the distribution of the temperature over the entire molding die within the solidification region and the solidification range, specifically, the control for the cooling rate within the molding die has a significant effect on the composition of the blended alloy composed of each of ultra-fine elements in the texture of the articles, control for the temperature distribution over the entire molding die upon coagulation of the molten metal is important.
However, since both of the male and female dies in the conventional molding die have been made of heat-resistant metal material, the cooling rate tends to be more rapid as a whole and the temperature of the die has to be raised in order to overcome the problem and the life of the die shortened, particularly if a pressure is applied thereto. It is thus difficult to control the temperature distribution for each of the portions in the molding die and fatal defects such as dents or sinks are caused at a thick-walled portion processing larger amount of heat where solidification is delayed as compared with other portions in that element are solidified without fusing to each other and intrude as deep seams to the inside of the articles and sometime form holes, as well as reduction in the durability of the molding die is also caused.
Further, if gases involved to the inside of the molding portion upon charging the molten metal in the casting process of this type are not rapidly degased upon charging, the back pressure in the molding portion is increased along with the charge of the molten metal to hinder the filling, thereby causing defective distribution of the molten metal to cause spiral defects in the articles, or the involved gases compressed upon charging of the molten metal and intruding near the surface thereof are expanded into swelled portions in the size of about rice grain upon mold-opening, which are left on the cast surface of the articles causing problems in view of the cast surface and appearance. Moreover, fine cast surface can not be obtained to cause surface defects failing to obtain fine cast surface with no cast creasings. Accordingly, rapid degasing of involved gases from inside of the molded portion has a significant effect for obtaining satisfactory cast surface with no surface defects from the surface of the articles.
Further, if gases evolved, paricularly, in the thick-walled portion with larger amount of heat upon solidification are not degased before the complete solidification the molten metal, gas sinks are liable to be caused in that portion causing the internal defect that impairs the strength. Accordingly, it is extremely important for rapidly and effectively eliminating gases before the complete solidification of the molten metal in order to obtain high quality and high strength articles with no defects in the inside thereof.
However, since degasing channels in the conventional molding dies are composed holes or grooves in communication with the atmospheric air, the gases involved into the molding part upon charging the molten metal are degased only while the charged molten metal flows through the degasing channels composed of the holes or the grooves and no effective degasing can be obtained. Further, no effective degasing can be attained also for the gases evolved upon solidification and, as a result, surface or internal effects are liable to be caused thereby bringing about severe problems in the art of this field.
Further, those insert component parts (movable hob, movable pin or the like) fitted to a male or female die of the molding die for applying pressure to the molten metal after charging are made of heat-resistant metal material, they can not withstand the thermal shocks undergoing from the high temperature molten metal at about 600-1650.degree. C. and, as a result, the surface of the inserts for urging the molten metal is damaged into spherical shape or causes cracking due to the repeatedly applied thermal shocks to result in significant effects on the quality and the dimensional accuracy of the articles.